The present invention relates to spherical tetragonal barium titanate particles and a process for producing the spherical tetragonal barium titanate particles, and more particularly, to spherical tetragonal barium titanate particles having an average particle diameter as small as 0.05 to 0.5 μm and a Ba/Ti ratio of 0.99:1 to 1.01:1.
With recent tendency toward miniaturization, high-performance and lightening of various electronic apparatuses, it has been required to improve characteristics of dielectric materials used in parts of these electronic apparatuses such as multi-layered ceramic capacitors or the like.
As known in the art, in many conventional multi-layered ceramic capacitors, there have been used barium titanate particles which are a perovskite compound having a high permittivity constant. The barium titanate particles, especially those used in the multi-layered ceramic capacitors, have been required to be free from agglomeration and have an excellent dispersibility as well as a high denseness, a high purity and excellent permittivity properties, since such particles are mixed with a binder upon the production of the multi-layered ceramic capacitors.
In order to satisfy the above requirements, the barium titanate particles have also been required to have a spherical shape and a sharp particle size distribution. Further, in the consideration of high permittivity properties, it has been required that the barium titanate particles have a ratio of Ba to Ti (Ba/Ti ratio) as close to 1.0:1 as possible, and exhibit a tetragonal crystal system.
As the conventional production process of the barium titanate particles, there are known a solid-phase reaction method of mixing a titanium compound with a barium compound and then calcining the resultant mixture at an elevated temperature of not less than 1,000° C., and a wet method of reacting barium and titanium with each other in a solution thereof.
However, the barium titanate particles obtained by the solid-phase reaction method not only have a large average particle diameter, but also are deteriorated in particle size distribution and fail to show a suitable particle shape for improved dispersibility since the particles are produced by pulverizing calcined particles. For this reason, the wet method has been more favorably used for the production of the barium titanate particles.
In the wet method, in order to obtain barium titanate particles having a Ba/Ti ratio as close to 1.0 as possible, it is necessary to add an excess amount of Ba to Ti. As to this fact, in “Journal of Japan Chemical Society”, No. 7, p.1155 (1974), it is apparently described that “The use of a relatively large amount of Ba2+is needed to produce BaTiO3 having a Ba/Ti ratio of 1. The studies have showed that a mixture to be reacted is required to have a Ba/Ti ratio of 8 in order to produce barium titanate having a Ba/Ti ratio of 1. When the Ba/Ti ratio in the mixture is less than 8, the obtained barium titanate is insufficient in barium content”.
In addition, in order to obtain tetragonal barium titanate particles, it is required to transform cubic particles into tetragonal ones by heating the former particles at an elevated temperature of not less than 800° C. However, when heat-treated at such an elevated temperature, the particles tend to be sintered together, and the obtained barium titanate particles tend to show a polygonal shape and a poor particle size distribution.
Conventional wet-reaction processes for producing barium titanate particles are described, for example, in Japanese Patent Application. Laid-Open (KOKAI) Nos. 61-31345(1986) and 62-72525(1987), Japanese Patent No. 2,999,821 and Japanese Patent Application Laid-Open (KOKAI) Nos. 5-330824(1993) and 8-119745(1996). Further, in Japanese Patent Application Laid-Open (KOAKI) Nos. 61-111957(1986) and 7-330427(1995) and Japanese translation of International Patent Application Laid-Open (KOHYO) No. 2000-509703, it is described that barium titanate particles are coated with silica or the like.
At present, it has been strongly required to provide barium titanate particles satisfying the above requirements. However, such barium titanate particles have not been obtained until now.
That is, in the method described in Japanese Patent Application Laid-Open (KOKAI) No. 61-31345(1986), after producing barium titanate particles, unreacted barium is insolubilized to ensure the production of barium titanate particles having a Ba/Ti ratio of 1.00. However, the thus obtained barium titanate particles are in the form of a mixture of cubic barium titanate and other Ba compounds. Therefore, in order to transform the cubic barium titanate into a tetragonal barium titanate as a single substance, it is necessary to conduct the transformation at a very high temperature as used in the conventional solid-phase reaction. Further, the thus-produced tetragonal barium titanate particles tend to be deteriorated in particle size distribution.
In Japanese Patent Application Laid-Open (KOKAI) No. 62-72525(1987), there is described the method of dissolving a barium compound in a titanium tetrachloride solution and then adding an aqueous alkali solution to the resultant solution to hydrothermally produce barium titanate particles. However, the calcined product of the obtained barium titanate particles does not have a single structure and, therefore, fails to show high permittivity properties, as described in Comparative Examples hereinafter.
Also, in Japanese Patent No. 2,999,821, there is described the method of reacting an excess amount of barium with titanium to produce barium titanate particles, and then after calcining, washing the resultant barium titanate particles with an acid to remove the excess barium therefrom. However, the obtained particles have a rectangular parallelopiped shape. Further, since barium contained in the barium titanate crystal tends to be eluted out by the acid-washing, it is difficult to stably control the Ba/Ti ratio. In addition, the acid-washing tends to cause the deterioration in crystal structure of the surface of the barium titanate particles.
In Japanese Patent Application Laid-Open (KOAKI) No. 5-330824(1993), there is described the method of subjecting a titanium compound and a barium compound to wet-reaction by adding an aqueous hydrogen peroxide solution thereto. However, the obtained barium titanate particles exhibit a cubic crystal system and, therefore, must be calcined for transforming into tetragonal barium titanate particles. Further, in the paragraph [0071] of this KOKAI, it is described that “tetragonal barium titanate is obtained by calcining the cubic particles at a temperature of 900 to 1,300° C. In this case, when the calcining temperature is low and the calcined particles have a large particle diameter, the obtained particles have a spherical shape. On the contrary, when the particle size is small and the calcining temperature is high, the obtained particles are phase particles having a rectangular parallelopiped shape”. Thus, in the case of barium titanate particles having a small average particle diameter, especially those having an average particle size as fine as not more than 0.5 μm, it is difficult to obtain spherical tetragonal barium titanate particles.
Further, as described in Comparative Examples later, when the cubic barium titanate having a Ba/Ti ratio of 1.02, which was produced by water-washing, filtering and drying the wet-reaction product according to the method described in Example 5 of Japanese Patent Application Laid-Open (KOAKI) No. 5-330824(1993), was calcined at a temperature of 1020° C., and the obtained calcined particles were measured by X-ray diffraction method, it was confirmed that a peak attributed to substance other than BaTiO3 (probably BaTi3O7) was observed. Thus, the obtained calcined particles are not phase particles and, therefore, fail to show excellent permittivity properties.
In Japanese Patent Application Laid-Open (KOAKI) No. 8-119745(1996), there is described the method of subjecting a mixture of barium hydroxide and titanium hydroxide to hydrothermal reaction to obtain barium titanate particles. However, the obtained particles exhibit a cubic crystal system. Further, when such cubic particles are calcined, the obtained particles are not single crystal particles and, therefore, are deteriorated in permittivity properties, as described in Comparative Examples later.
In Japanese Patent Application Laid-Open (KOAKI) No. 61-111957(1986), there is described the method of calcining a mixture containing a low-melting material composed of an oxide of an element selected from the group consisting of Bi, B, Pb and W, and cubic barium titanate. However, upon calcination of the cubic barium titanate, the low-melting material is reacted with the cubic barium titanate, thereby partially forming a solid solution therebetween. Thus, the obtained particles tend to be deteriorated in permittivity properties.
In Japanese Patent Application Laid-Open (KOAKI) No. 7-330427(1995), there is described the method of coating the surface of barium titanate particles with alumina and calcining the resultant coated particles together with a glass component added thereto. However, in this KOKAI, it is also described that when the amount of the glass component added is small, the obtained particles suffer from generation of voids due to the reaction and are deteriorated in permittivity properties. Thus, in Japanese Patent Application Laid-Open (KOKAI) No. 7-330427(1995), there is no suggestion that the permittivity properties of barium titanate particles can be improved merely by addition of a small amount of silica.
Further, in Japanese translation of International Patent Application Laid-Open (KOHYO) No. 2000-509703, there are described barium titanate particles whose surface is coated with metal oxide, hydrated metal oxide, metal hydroxide or an organic acid salt. However, the object of this KOHYO is to improve the dispersibility of particles by controlling the surface coat formed thereon and the particle size distribution thereof. Therefore, there is neither suggestion as to anti-sintering effect nor teachings as to crystal system and crystallinity.
In addition, in Japanese Patent No. 3,146,961, it is described that a Si component is incorporated into barium titanate particles. However, there is no suggestion as to crystal system. Also, since the Si component forms a solid solution with the barium titanate, the obtained particles tend to be deteriorated in permittivity properties because of reduction in components contributing to improvement of the permittivity properties.
As a result of the present inventors' earnest studies for solving the above problems, it has been found that by adding an aqueous barium salt solution to a titanium hydroxide colloid in the presence of carboxylic acid in an amount of 1 to 60 mol % based on the mole of barium contained in the aqueous barium salt solution, thereby producing barium titanate starting particles; hydrothermally treating a resultant reaction solution containing the barium titanate starting particles at a temperature of 100 to 300° C., thereby obtaining spherical cubic barium titanate particles; and after washing, calcining the spherical cubic barium titanate particles at a temperature of 500 to 1,200° C., the obtained spherical tetragonal barium titanate particles are free from agglomeration, and can exhibit an excellent dispersibility as well as a high denseness, a high purity and excellent permittivity properties. The present invention has been attained on the basis of this finding.